The disclosure relates to a hydraulic machine having an adjustable displacement volume in accordance with the preamble of Patent claim 1, to a transmission arrangement having the hydraulic machine according to Patent claim 8, and to a method for controlling the transmission arrangement according to Patent claim 11.
A transmission arrangement of the generic type has a hydraulic motor, by which a transmission input of a mechanical, shiftable transmission is driven. In order that a gear change can take place reliably, the hydraulic motor, that is to say the input of the manual transmission, has to be free from torque during the shift. After a shift request, there is therefore always the question as to the time at which said freedom from torque is achieved.
Since the torque of a hydraulic machine is proportionally dependent on its displacement volume, solutions having a detection unit for the displacement volume are known from the prior art. If the value zero is detected via said detection unit, this is synonymous with the freedom from torque. The detection unit is frequently configured as a zero position switch.
It is a disadvantage of said zero position switches that the provision thereof on or in the hydraulic machine represents additional complexity. For example, costs accrue for the switch/sensor, the cables, plugs and for the respective integration into different housing variants of the hydraulic machine, and for the maintenance of the zero position switch. Operation is possibly no longer possible in the case of a defect of the switch. A restricted availability of the traction drive or of the vehicle can thus occur. In some cases, it is also very difficult to position the zero position switch in or on the hydraulic machine.
As an alternative, time-based actuation means are known from the prior art, in the case of which an adjustment time of the displacement volume of the hydraulic machine, after the expiration of which adjustment time the displacement volume is zero, including a safety reserve, is estimated. It is a disadvantage of this that the great variance in real operation, for example of the pressure medium temperature, the machine temperature, the adjustment pressure, the working pressure and component-related series variation, is not taken into consideration. In the case of the method of time-based estimation of the load-free or torque-free state, it can occur, for example in the case of technical defects, that the hydraulic machine still has (“provides”) a finite displacement volume and therefore a torque after expiration of the time period, since the adjustment dynamics of the hydraulic machine are approximated by way of a time model which does not provide for defect. If the shift then nevertheless occurs, that is to say under load, this can lead to damage of the transmission up to its failure. If, in contrast, the time-based estimation is parameterized conservatively in order to prevent damage of this type, unnecessarily long shift times can occur which can then possibly bring the vehicle into a critical situation in the case of a traction force interruption of said vehicle, which traction force interruption accompanies the shift operation.
The specification of DE 10 2015 215 013 A1 discloses a solution, in the case of which the torque of the second hydraulic machine which is coupled to the manual transmission is regulated to zero or close to zero for the gear change. This takes place via the regulation of the differential pressure across the working connectors of the second hydraulic machine. To this end, it is proposed in one variant to equip the first hydraulic machine of the hydraulic circuit with a differential pressure regulator. In another variant, the regulation of the differential pressure takes place indirectly via the regulation of the control pressure, with which the adjustment device of the first hydraulic machine is loaded in order to adjust its displacement volume. As a consequence, the freedom from torque or the virtually torque-free state of the second hydraulic machine is ensured solely via the first hydraulic machine. Although this also makes the use of an inexpensive fixed displacement machine as a hydraulic motor possible, the regulating quality and regulating dynamics can cause problems.